The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Electronic devices such as computers, laptops, personal video recorders (PVRs), MP3 players, game consoles, set-top boxes, digital cameras, etc., often need to store a large amount of data. Storage devices such as hard disk drives (HDDs) may be used to meet these storage requirements.
Referring now to FIG. 1, a hard disk drive (HDD) 10 includes a hard disk assembly (HDA) 50 and a HDA printed circuit board (PCB) 14. The HDA PCB 14 comprises a buffer module 18 that stores data associated with the control of the HDD 10. The buffer module 18 may employ SDRAM or other types of low latency memory. A processor 22 is arranged on the HDA PCB 14 and performs processing that is related to the operation of the HDD 10.
A hard disk controller (HDC) module 26 communicates with the buffer module 18, the processor 22, a spindle/VCM (voice coil motor) driver module 30, and an input/output interface module 24. The input/output interface module 24 can be a serial interface module, a parallel interface module, a serial Advance Technology Attachment (ATA) interface module, a parallel ATA interface module, etc.
Additionally, the HDC module 26 communicates with a read/write channel module 34. During write operations, the read/write channel module 34 encodes data that is to be written by a read/write device 59. The read/write channel module 34 processes data for reliability using error correction coding (ECC), run length limited coding (RLL), etc. During read operations, the read/write channel module 34 converts an analog output of the read/write device 59 into a digital signal. The digital signal is then detected and decoded using known techniques to recover the data written on the HDD 10.
The HDA 50 includes one or more circular recording surfaces called platters 52 that are used to store data. The platters 52 include a magnetic coating for storing data in terms of magnetic fields. The platters 52 are stacked on top of one another in the form of a spindle. The spindle comprising the platters 52 is rotated by a spindle motor 54. Generally, the spindle motor 54 rotates the platters 52 at a fixed speed during read/write operations. The spindle/VCM driver module 30 controls the speed of the spindle motor 54.
One or more actuator arms 58 move relative to the platters 52 during read/write operations. The spindle/VCM driver module 30 also controls the positioning of the actuator arm 58 by using mechanisms such as a voice coil actuator, a stepper motor, etc. For example, a voice coil motor (VCM) 57, which is controlled by the spindle/VCM driver module 30, may be used to control the positioning of the actuator arm 58.
The read/write device 59 is located near a distal end of the actuator arm 58. The read/write device 59 includes a write element such as an inductor (not shown) that generates a magnetic field. The read/write device 59 also includes a read element (such as a magneto-resistive (MR) element, also not shown) that senses magnetic field on the platters 52. The HDA 50 includes a preamp module 60, which amplifies analog read/write signals.
When reading data, the preamp module 60 amplifies low-level signals from the read element and outputs the amplified signal to the read/write channel module 34. While writing data, a write current is generated that flows through the write element of the read/write device 59. The write current is switched to produce a magnetic field having a positive or negative polarity. The positive or negative polarity is stored on the hard drive platters 52 and is used to represent data.
Referring now to FIG. 2, data is typically written on the platters 52 in concentric circles called tracks 70. The tracks 70 are divided circumferentially into multiple sectors 72. A circumferential length 74 of sectors 72 decreases as the diameter of the tracks 70 decreases towards the center of the platters 52.
Before performing a read/write operation on a sector 72 of a track 70, the read/write device 59 locks onto the track 70 by referring to positioning information called servo. Servo is generally prewritten on the platters 52 and provides the positioning information to allow the read/write device 59 to read and write data at correct locations on the platters 52.
Data can be correctly read and/or written if servo is written accurately. Traditionally, servo is prewritten using a special servo writing apparatus when a disk drive is manufactured. The servo writing apparatus typically includes a precision actuator used to position an actuator arm. The servo writing apparatus also typically uses an external clock-head to position servo wedges at a predetermined offset. Accordingly, the servo writing apparatus can be expensive and may increase the cost of manufacturing HDDs.
Additionally, traditional servo writing methods become less practical as the track density of disk drives increases. Track density is generally expressed in terms of number of tracks per inch of the platters 52. The track density may increase due to two reasons: decreasing diameter of platters 52 and increasing storage capacity of disk drives. In recent years, the physical size of disk drives is decreasing, and the storage capacity of disk drives is increasing. This is because of growing demand for smaller devices such as palmtops, etc., that use disk drives that are compact in size and high in storage capacity.
Accordingly, HDDs increasingly use self-servo-write (SSW) methods to write their own servo instead of using external servo writing apparatus. Disk drives that use SSW methods to write servo utilize the same read/write heads that are used to read/write regular data. When writing servo using SSW methods, the heads typically lock onto reference servo sectors (RSS) that are prewritten on the platters 52 either concentrically or in the form of spirals.
Referring now to FIGS. 3A-3B, a SSW module 28 may communicate with the processor 22 and the HDC module 26 as shown in FIG. 3A. The SSW module 28 may generate control signals to write servo on platters 52. For example, the SSW module 28 may generate control commands that control movement of the actuator arm 58 during servo writing. The HDC module 26 and the spindle/VCM driver module 30 may implement the control commands during SSW. The SSW module 28 may generate a servo pattern that is written on the platters 52 using the read/write device 59. Additionally, the SSW module 28 may utilize the processor 22 to verify the servo pattern by performing read-after-write operations, etc.
During SSW, the platters 52 may rotate in direction A, and the actuator arm 58 may move in direction B as shown in FIG. 3B. A disk drive typically uses motion delimiters called crashstops that prevent the actuator arm 58 from moving beyond safe limits. For example, the VCM 57 causes the actuator arm 58 to move between crashstop 55 and crashstop 53 while the read/write device 59 writes a servo spiral 80 between tracks 70.
When spirals 80 are written, the actuator arm 58 is accelerated from a stationary position to a predetermined velocity by applying current to the VCM 57. Spirals 80 are written accurately when the actuator arm 58 moves at the predetermined velocity. Thus, spirals 80 that are written while the actuator arm 58 accelerates may be imperfect, that is, the spirals 80 may have wrong slopes. The portion of platters 52 where spirals 80 are imperfectly written may be unusable, which may reduce storage capacity of HDDs.